In the field of non-permanent floor panel attachment, a nut and bolt combination has almost always been employed. A simple method involves directly passing a threaded bolt (or stud) through the front side of a passageway between the floor panel and underlying structure, and attaching a likewise threaded nut located on the backside of the passage to the bolt (stud) (in the case of a threaded stud, nuts are positioned at the front and backside of the passage to engage with the stud). Torque is then applied to the nut(s) with respect to the bolt (stud) in order to tighten the floor panel to the underlying structure.
A more sophisticated installation uses a potted-in insert installed in an enlarged passageway in the floor panel, the insert having an internal passageway that cooperatively receives a stud or bolt in general—a countersunk headed machine screw being preferred. This is particularly useful when the floor panels are made up of a honeycombed composite material that is easily distorted via torque and compression, and the insert is bonded to said material. Usual installation involves an insert with topside that resides near the front side of the passageway, and a bottom side that has an enlarged bonding flange and resides near the interface between the floor panel and the underlying structure. The topside of the insert is either swaged or not swaged to form fit the front side of the floor panel, and the enlarged bonding flange has a bonding surface that facilitates the bonding of the insert to the floor panel. However, any combination of bonding flanges and swaging operations may be used to install the insert into the floor panels, including a two-piece insert that has two bonding surfaces, one on each face of the floor panel.
An even more elaborate and industry preferred method for attaching floor panels to underlying structure involves using the aforementioned potted-in insert, the insert being installed in a honeycomb or similar floor panel; attaching a nut-plate element to the backside of the passageway in the underlying structure; and a countersunk headed screw being inserted through the passage way in the insert, and being torqued with respect to the nut-plate element, the nut-plate element being held by various methods to the underlying structure. Said torque rotates the countersunk screw with respect to the nut-plate element, thus causing subsequent tightening of the floor panel to the underlying structure.
The nut element of the prior art has many variations, two of the more popular being a nut-plate and a clip-nut. The nut-plate variation has a nut element being captured in a formed plate—said nut is captured such that it is centrally located over a center hole in the plate and offers small lateral misalignments. In addition, the plate which holds the nut element also has a least two additional holes located near the edges of the plate to serve as attachment points to the structure; the entire nut-plate being riveted through those holes to the underlying structure with the center hole located in line with the passageway in the underlying structure.
The clip-nut variation of the nut-plate element has a nut being captured or formed in a “U” shaped plate. The opening of the “U” shaped plate is sized to match the structure it is mounted on and the nut element is placed to be in line with the structure. The nut is captured and held tightly on the bottom outer portion of the “U” shape by a formed return tab. The return tab is structured so the nut is held in alignment over the through hole or passageway, and held from rotating with respect to the plate element.
Current manufacturing methods in the aerospace industry commonly use nut-plates and clip-on nut-plates to fasten panels to underlying structure, as described above. However, these existing methods often are accompanied by substantial detrimental consequences that result in costly reworks and delays.
In the existing manufacturing process, riveted-on nut-plates can be used in places where clip-on nut-plates do not have adequate throat depth to span the distance between the passageway and the edge of the underlying structure. However, the installation of riveted on nut-plates is time consuming and requires expensive specialized equipment. One installation can typically take 15 minutes to complete as specialized equipment must drill the main center hole in the underlying structure plus two additional smaller holes through which rivets are installed. Both rivets act as a torque reacting element and hold the nut-plate to the structure when not engaged in fastening. The nut-plate must then be riveted to the underlying structure using still more specialized equipment. Special care must be taken during this process not to misalign or mis-locate the rivets or the rivet holes as such misalignment can result in a variety of problems, e.g., replacement of the entire structure, an extremely expensive process.
In addition to the foregoing, the lateral float of a riveted nut-plate can be eliminated in later coating processes commonly used to prevent corrosion. In effect, the nut can be frozen in an out of alignment condition by sealants or corrosion resistant coatings, thus preventing the bolt from engaging the nut element during subsequent torquing operations.
Clip-on nut-plates are preferred over the riveted-on variety, as they do not require specialized equipment and processes. As described above, they are simply installed sideways onto the structure—the open end of the “U” shape is bent together to form a type of spring with clamping force and is inserted laterally onto the structure so that the structure is sandwiched between the top and bottom of the “U” element. The closed side of the “U” element serves as the torque reacting feature. The slightly bent “U”—now open after insertion over the structural element—provides a slight spring like clamping to hold it in position when not engaged. The top of the “U” portion has a bent down locator tab, which axial locates the clip-on nut-plate to the hole in the structure. This locator tab is usually undersized, thus allowing the nut-plate to move laterally within the range determined by the size difference between the hole in the structure and the locator tab.
Unfortunately, the locator tab has a sharp edge that scratches the surface of the underlying structure every time a clip nut is removed or installed, even when the underlying structure is treated with anti-corrosion coatings. The associated scratches resulting from clip nut installation and removal provide corrosion starting points. In addition, the back of the “U”-shape, which acts as a torque reactor, nicks and rubs against the underlying structure, further providing corrosion starting points.
It is also important to note that while each nut-plate design has unique problems associated with their use, they also share common disadvantages as described below. Both designs utilize thread-locking devices, which are required to provide a certain level of resistance to vibration forces that tend to loosen fasteners over a period of time during the operation of the aircraft. These thread locking devices are most commonly mechanical in nature and constitute mechanically deforming the nut. The purpose of the thread distortion is to providing a continuous preload on the threads to resist vibration. Therefore, additional torque must be applied to overcome the thread-locking feature while applying the specified clamping force.
It is also difficult to control the accuracy of the thread-locking measures. Sometimes the deformation is too extreme, causing a stud to freeze within the nut upon installation. Other times, the deformation is inadequate, and the fastener vibrates loose over time.
Another major disadvantage of these types of nut-plates share is that they require access to both sides of the structure—they are not true blind side fasteners. For instance, if an individual nut element is damaged or faulty (too much distortion on the thread-locking devices) and must be replaced during installation of a floor panel, the entire floor panel and all subsequently installed studs must be removed in order to gain access to the faulty nut-plate. Also, additional labor is required to install each nut-plate, then place the floor panel into position, then attach and torque the studs into position.
Finally, the current nut-plate designs have some alignment difficulties. The nut element is not designed to find the installation center of the stud. The limitation sometimes results in damaging the threads on either the nut element or the stud—cross threading.